Interactions between β-cyclodextrin and an amino acid-based anionic gemini surfactant derived from cysteine

The interaction between beta-cyclodextrin (beta-CD) and an amino acid-based anionic gemini surfactant derived from cysteine (C(8)Cys)2 was studied by three independent techniques: electrical conductivity, UV-Vis spectral displacement technique using phenolphthalein as probe, and H-1 NMR spectroscopy. The data obtained indicated the formation of a 1:1 inclusion complex between beta-CD and the gemini surfactant studied and allowed for the determination of the binding constant, K-1, by considering this stoichiometry. Electrical conductivity, spectral displacement technique, and NMR chemical shift measurements, obtained for aqueous beta-CD-surfactant systems, yielded consistent K-1 values in the order of 10(2) dm(3) mol(-1), typical of a weakly bound beta-CD-surfactant complex. The influence of the presence of the inclusion complex on the micellization process of the gemini surfactant has also been studied and the apparent critical micelle concentration (cmc*) has been obtained. Increasing beta-CD concentration was found to shift the cmc* to higher values, as complexed surfactant monomers are not available to form micelles and aggregation takes place only when all beta-CD cavities are occupied. (C) 2011 Elsevier Inc. All rights reserved.